Machinery's Handbook, 31st Edition
Bevel Gearing 2259 bevel, and hypoid gears, clearance must be provided for face-mill type cutters; front and rear hubs should not intersect the extended root line of the gear or they will interfere with the path of the cutter. In addition, there must be enough room in the front of the gear for the clamp nut that holds the gear on the arbor, or in the chuck, while cutting the teeth. The same considerations must be given to straight bevel gears that are to be generated using a circular-type cutter instead of reciprocating tools. Mountings for Bevel Gears.— Rigid mountings should be provided for bevel gears to keep the displacements of the gears under operating loads within recommended limits. To align gears properly, care should be taken to ensure accurately machined mountings, properly fitted keys, and couplings that run true and square. As a result of deflection tests on gears and their mountings, and having observed these same units in service, the Gleason Works recommends that the following allowable deflections be used for gears from 6 to 15 inches (15.24–38.10 cm) in diameter: neither the pinion nor the gear should lift or depress more than 0.003 inch (0.076 mm) at the center of the face width; the pinion should not yield axially more than 0.003 inch (0.076 mm) in either direction; and the gear should not yield axially more than 0.003 inch (0.076 mm) in either direction on 1 to 1 ratio gears (miter gears), or near miters, or more than 0.010 inch (0.25 mm) away from the pinion on higher ratios. When deflections exceed these limits, additional problems are involved in obtaining satisfactory gears. It becomes necessary to narrow and shorten the tooth contacts to suit the more flexible mounting. These changes decrease the bearing area, raise the unit tooth pressure, and reduce the number of teeth in contact, resulting in increased noise and the danger of surface failure as well as tooth breakage. Spiral bevel and hypoid gears in general should be mounted on antifriction bearings in an oil-tight case. Designs for a given set of conditions may use plain bearings for radial and thrust loads; maintaining gears in satisfactory alignment is usually more easily accom plished with ball or roller bearings. Bearing Spacing and Shaft Stiffness: Bearing spacing and shaft stiffness are extremely important if gear deflections are to be minimized. For both straddle mounted and overhung mounted gears the spread between bearings should never be less than 70 percent of the pitch diameter of the gear. On overhung mounted gears the spread should be at least 2 1 ∕ 2 times the overhang and, in addition, the shaft diameter should be equal to or preferably greater than the overhang to provide sufficient shaft stiffness. When two spiral bevel or hypoid gears are mounted on the same shaft, the axial thrust should be taken at one place only and near the gear where the greater thrust is developed. Provision should be made for adjusting both the gear and pinion axially in assembly. Details on how this may be accomplished are given in the Gleason Works booklet, “Assembling Bevel Gears.” Cutting Bevel Gear Teeth.— A correctly formed bevel gear tooth has the same sectional shape throughout its length, but on a uniformly diminishing scale from the large to the small end. The only way to obtain this correct form is by using a generating type of bevel gear cutting machine. This accounts, in part, for the extensive use of generating type gear cutting equipment in the production of bevel gears. Bevel gears too large to be cut by generating equipment —100 inches (254 cm) or over in diameter—may be produced by a form-copying type of gear planer. A template or former is used to mechanically guide a single cutting tool in the proper path to cut the profile of the teeth. Since the tooth profile produced by this method is dependent on the contour of the template used, it is possible to produce tooth profiles to suit a variety of requirements. Although generating methods are to be preferred, there are still some cases where straight bevel gears are produced by milling. Milled gears cannot be produced with the accuracy of generated gears and generally are not suitable for use in high-speed applica tions or where angular motion must be transmitted with a high degree of accuracy. Milled gears are used chiefly as replacement gears in certain applications, and gears which are
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